Of thermoplastic resins, aromatic vinyl resins occupy a relatively large proportion of uses. A representative aromatic vinyl resin is copolymers of an aromatic vinyl monomer and an acrylic ester monomer. Such resins having various compositions are put on the market. However, in general they are not sufficient for market demands in strength. Accordingly, various manners to improve the impact strength have been adopted. For example, JP-A-52-8095 and JP-B-55-7849 disclose improving the impact resistance by dissolving a rubber in monomers for constituting an aromatic vinyl resin and subjecting the monomers to bulk polymerization or suspension polymerization. JP-A-54-153884 discloses incorporating an aromatic vinyl resin with a rubber-containing multi-layer structure obtained by emulsion polymerization of a copolymerizable vinyl monomer in the presence of a rubber component made of butyl acrylate or butadiene. Besides, JP-A-55-157642 and JP-A-56-70043 disclose mixing an aromatic vinyl resin with a block copolymer containing a rubber component as represented by styrene-butadiene block copolymer. However, the former is restricted in the content of rubber from the viewpoint of solubility and, therefore, the strength is not sufficiently improved. In case of rubber-containing polymers prepared according to the latter methods, known rubber-containing polymers introduce a problem, since if it is attempted to secure the impact resistance, they must be used in a large amount such that other properties are no longer maintained. That is to say, if the amount of a rubber is increased to the extent that a desired impact resistance can be secured, characteristics such as rigidity, heat resistance and transparency that aromatic vinyl resins originally possess are lowered, so the known rubber-containing polymers are hard to be put to practical use. It is expected that if an impact strength can be imparted to aromatic vinyl resins without impairing the advantages of base resins, they could be used in wider purposes.
Accordingly, it is an object of the present invention to impart an impact strength to aromatic vinyl resins without impairing the properties such as rigidity and transparency thereof.
A further object of the present invention is to provide an aromatic vinyl resin composition having well-balanced properties such as impact resistance, rigidity, heat resistance and processability.